Electrocatalytic oxidation of methanol and ethanol: a comparison of

3284

Langmuir 1993,9, 3284-3290

Electrocatalytic Oxidation of Methanol and Ethanol: A Comparison of Platinum-Tin and Platinum-Ruthenium Catalyst Particles in a Conducting Polyaniline Matrix Christopher T. Hable and Mark S. Wrighton' Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received October 1,1992@ A method is described for preparing polyaniline/Pt-Ru assemblies that are effective catalysts for electrochemicaloxidation of MeOH or EtOH in aqueous H2S04. The Pt-Ru particles have been deposited into polyaniline by electrochemical deposition from aqueous H2S04 containing Pt(1V) and Ru(II1) from addition of K2PtCb and K2RuCl~rHz0,respectively. The activity of the polyaniline/Pt-Ru assemblies for MeOH or EtOH oxidation is higher than that of polyaniline-coated electrodes modified with Pt alone. For the polyaniline/Pt-Rusystem,high catalytic activity is observed for Pt-Ru particles that are codeposited from solutionscontaining from -0.5 to 2mM Ru(III) when a 3mM Pt(IV) solutionis used. Characterization of the electrodes by SEM showsthat the codeposition procedure yields roughly spherical catalyst particles 300-400 nm in diameter dispersed throughout the polyaniline. XPS analysis shows Ru to be present in two different oxidation states, most likely Ru(0) and Ru(IV). The Pt-Ru catalyst is compared to a Pt-Sn catalyst which can also be dispersed in a polyaniline matrix. For electrodes prepared from 3 mM Pt(1V) with an appropriate amount of Sn(1V)or Ru(III), the Pt-Ru catalyst is better for MeOH oxidation than the Pt-Sn catalyst, because it is more reproducible. Both catalysts show a significant (and similar) temperature dependencefor MeOH or EtOH oxidation. For EtOH oxidation the Pt-Sn catalyst is superior to the Pt-Ru catalyst, and the activity of Pt-Sn at low potentials, where polyaniline is nonconducting, is limited by the resistivity of the polyaniline film. For EtOH oxidation the initial product is acetaldehyde which,unfortunately, degradesthe conductivity of polyaniline. Oxidation of MeOH occurs at more positive potentials and does yield C02, though formaldehyde,an intermediateproduct, in aqueous acid also degrades the conductivity of polyaniline.

In this paper we show that Pt-Ru binary catalyst particles can be incorporated into a conducting polyaniline matrix and present a comparison of electrochemical oxidation of MeOH and EtOH at polyaniline/Pt-Sn and polyaniline/Pt-Ru catalyst assemblies. We show that polyaniline is a viable support material for different types of electrocatalyst particles and demonstrate the versatility of electrocodeposition as a method for incorporating binary metal catalyst particles into a conducting polyaniline film. The polyaniline/Pt-Ru assembliesshow structural features that are similar to the polyaniline/Pt-Sn assemblies previously studied.' For electrodes prepared in a similar manner, the activity of polyaniline/Pt-Ru assemblies for MeOH oxidation is somewhat better than that of polyaniline/Pt-Sn. In contrast, the activity for EtOH oxidation is superior on the polyaniline/Pt-Sn in comparison to polyaniline/Pt-Ru. The results for the polyaniline supported catalysts show trends that are similar to MeOH or EtOH oxidation on Pt electrodes modified with Sn and Ru adatoms.2 The Pt-Ru electrocatalyst has been previously studied in a number of forms including electrodeposited Pt-Ru? Raney-type alloys,4 and chemically precipitated Pt-Ru imbedded in Teflon bonded electrodes5 and in a polymer electrolyte.6 The Pt-Sn electrocatalyst has been studied as an electrodeposit on Rh, Ir, and Pt and in a polymer electrolyte.6 However, the incorporation of Pt-

* Author to whom correspondence should be addressed.

* Abstract published in Advance ACS Abstracts, September 1, 1993. (1) Hable, C. T.; Wrighton, M. S. Langmuir 1991, 7, 1305. (2) Shibata, M.; Nagakazu,F.; Watanabe,M.; Mooto, S. Denki Kagaku 1988, 56, 774. (3) (a) Petrii, 0. A. Dokl. Phys. Chem. 1966,160,117. (b) Entina, V. S.; Petrii, 0. A. Sou. Electrochem. 1967, 3, 1107. (c) Petry, 0. A.; Podlivchenko, B. I.; Frumkin, A. N.; Lal, H. J.Electroanal. Chem. 1965, 10, 253.

Ru and Pt-Sn catalyst systems in a conducting polymer matrix offers the possibility of obtaining higher surface areas. The use of a polymer such as polyaniline, should allow use of thicker coatings of polymer and higher catalyst loadings, owing to the high conductivity of the polymer. Polyaniline is a particularly attractive material for a catalyst support because of its high surface area,loJ1 high conductivity,12and durability under conditions relevant to the operation of MeOH fuel cells employing aqueous acidic ele~trolytes.'~J~ The polyaniline/Pt-Ru and polyaniline/Pt-Sn assemblies have been studied by scanning electron microscopy (SEM)and X-ray photoelectron spectroscopy (XPS) and the resulting structure is in accord with the structure represented in Scheme I. As in the case of the Pt-Sn system, the overpotential for MeOH oxidation is lowered by the Pt-Ru catalysts compared to Pt alone in a (4) (a) Binder, H.; Kohling, A.; Sandstede, G. In Hydrocarbon Fuel Cell Technology; Baker, B. S., Ed.; Academic Press, Inc.: New York, 1965; pp 91-101. (b) Watanabe, M.; Motoo, S. J. Electroanal. Chem. 1975, 60, 267. (5) (a) Kennedy, B. J.; Smith, A. W. J.Electroanal. Chem. 1990,293, 103. (b) Goodenough, J. B.; Hamnet, A.; Kennedy, B. J.; Manoharan, R.; Weeks, S. A. J.Electroanul. Chem. 1988,240,133. (c) Swathirajan, S.; Mikhail, Y. M. J.Electrochem. SOC.1991,138, 1321. (6) (a) Aramata, A,; Masuda, M. J.Electrochem. SOC.1991,138,1949. (b) Aramata, A,; Kodera, T.; Masuda, M. J.Appl. Electrochem. 1988,18, 577.

(7) Cathro, K. J. J.Electrochem. SOC.1969,116, 1608. (8) Janasen, M.M. P.; Moolhuysen,J. Electrochim.Acta 1976,21,861. (9) Andrew, M. R.; Drury, J. S.; McNicol, B. D.; Pinnington, C.; Short,

R.T.J. Appl. Electrochem. 1976, 6.99.

(10) Carlin, C. M.; Kepley, L. J.; Bard, A. J. J.Electrochem. SOC.1986, 132, 353. (11) Huang, W.-S.; Humphrey, B. D.; McDiarmid,A. G. J. Chem.SOC., Faraday Trans. 1 1986,82, 2385. (12) Paul, E. W.; Ricco, A. J.; Wrighton, M. S. J. Phys. Chem. 1986, 89, 1441. (13) McNicol, B. D. J. Electroanal. Chem. 1981, 118, 71. (14) Hampson, N. A.; Willars, M. J.;McNicol, B. D. J.Power Sources 1979, 4, 191.

0743-7463/93/2409-3284$04.00/0 1993 American Chemical Society

Electrocatalytic Oxidation of MeOH and EtOH Scheme I. Incorporation of Catalyst into a Polyaniline Matrix Electrode

I ,

Pt-Sn or Pt-Ru Catalyst Particles I\

Polyaniline

pty

Langmuir, Vol. 9,No. 11, 1993 3285 +0.9 V from 20 to 100 mV/s until the desired coverage of polymer was obtained. Coverage was measured by determiningthe charge passed corresponding to the oxidation and reduction of the polyaniline in 0.5 M H2S04 for scans between -0.2 and +0.9 V vs SCE. Typical amounts of charge were 3 X 10-1 to 7 X 10-' C/cm2of electrode area. Binary catalyst particles were incorporated into the polyaniline matrix by cycling the polyaniline modified electrodes in a solution containing Pt(1V) and Sn(1V) or Pt(1V) and Ru(II1) complexes between +0.5 V and -0.3 V at 50 mV/s. The Pt-Sn catalyst was deposited from solutions of 0.5 M H2SO4 containing 3 mM KzPtCb and 7 mM SnCL.5H20. The Pt-Ru catalyst was deposited from solutions of 0.5 M H2S04 containing 3 mM K2PtCb and 1.3 mM K2RuClb-xH20 except where indicated. Active catalysts can be made from more concentrated solutions of the precursors as long as the ratios of the precursors are kept constant. Solutions containing higher concentrations of K2PtCb and SnCL05H20 are more stable and give high catalytic activity more reproducibly. From measurements of cathodic charge passed corresponding to reduction of Pt(1V)to Pt(O),we can estimate the amount of Pt deposited into the polyaniline film; typical values were 0.25-1.0 mg of Pt/cm2 of electrode area. Microscopy and Spectroscopy. SEM was done on a Hitachi Model S-800electron microscope. XPS spectra were obtained on a Surface Science Instruments Model SSX-100 spectrometer using monochromatic A1 Ka radiation and operating at -1 X 10-8 Torr. A 1-mm spot size was used for all XPS analyses. Product Analysis. C02 production was monitored by flowing a stream of Ar over the electrolysis cell and into an aqueous BaOH trap to precipitate CO2 as BaC03. For product analysis by 13CNMR the electrolysis was run in 0.5 M H2S04 in H20, but prior to NMR analysis a small amount ( l e 2 0 % ) of a deuterated solvent was added to provide a lock signal.

-

Sn(lV) or Ru(lll)+ Reduce

polyaniline matrix.15J6 Polyaniline/Pt modified electrodes have also been studied for formic acid 0xidati0n.l~ Pd and combinations of metals together with polyaniline have been studied for methane oxidation18and polyaniline/Pd assemblies have been studied for H+ and 0 2 red~cti0n.l~ However, oxidation of MeOH at Pt-Sn or Pt-Ru catalysts supported on polyaniline is unique in that the maximum activity for electrocatalytic oxidation of MeOH at both the Pt-Sn and Pt-Ru catalyst systemsoccurs in a potential region where polyaniline is most conductive.12 Combinations of metals supported on polyaniline and other conducting polymers have been used as anodes and cathodes in a MeOH/air fuel Both the polyanilinel Pt-Sn and polyaniline/Pt-Ru catalyst assemblies show a significant thermal activation for MeOH oxidation, the current at +0.3 V vs SCE increasing almost an order of magnitude from 0 to 50 "C in 0.5 M H2S04 with 20% by volume of added alcohol. Experimental Section Electrochemistry. Electrochemical experiments were run using a PAR Model 173 potentiostat with a PAR Model 175 programmer. Current-voltage curves were recorded on a Kipp and Zonen 90B XY or XYY' recorder. Cyclic voltammetry was performed in a two-compartmentcell using a saturated calomel (SCE)reference electrode and a Pt gauze counter electrode. Where the data have been normalized to electrode area, the area indicated refers to the geometric area of the electrode (i.e. the original area of the electrode prior to any modification). Glassy carbon (GC) electrodes(3mm diameter) were obtained from BioanalyticalSystems. GC electrodeswere polished initially with 9-pm to 1-pm diamond paste. Before each experiment electrodes were polished again with 0.3-pm alumina, followed by sonication in H20. Substrates for SEM, XPS, and AES were either polished glassy carbon sheet (Electrosynthesis)or 1pm thick e- beam evaporated Au on a polished Si wafer (Silicon Sense)with a Cr (50A) adhesion layer. K2PtCb (Strem or Alfa), SnCL-5H20 (Strem), K2RuCl5-xH20 (Strem), H2S04 (Mallinckrodt), and HPLC grade H20 (Omnisolve, EM Science),were used as received. Aniline (Aldrich) was distilled and stored under Ar. Electrode Modification. Electrodes were derivatized with polyaniline by electropolymerization12of 0.1 to 0.5 M aniline in 1.0 M H2SO4. Electrodes were cycled repeatedly from -0.2 to ~~~~~~~

(15)Kost, K. M.; Bartak, D. E.; Kazee, B.; Kuwana, T. Anal. Chem. 1988,60,2379. (16)Ocon Esteban, P.;Leger, J.-M.; Lamy, C.; Genies, E. J. Appl. Electrochem. 1989,19,462. (17)(a) Gholamian, M.; Sundaram, J.; Contractor, A. Q. Langmuir 1987,3,741.(b)Gholamian, M.; Contractor, A. Q. J.EZectroanaZ. Chem. 1990,289,69. (18)Leone, A.; Marino, W.; Scharifker, R. B. J. Electrochem. SOC. 1992,139,438. (19)Scharifker, B.;Yepez, 0.;de Jesus, J. C.; Ramirez de Agudelo, M. M. Ger. Offen. DE 4 040 835,1991;Chem. Abstr. 1991,115,654. (20)Naarmann, H.; Sterzel,H.-J. Ger. Offen. DE 3912735,1990;Chem. Abstr. 1991,114,184.

-

Results and Discussion SurfaceAnalysisof Polyaniline/Pt-Ru Assemblies. Figure 1 shows SEM micrographs of Pt-Ru catalyst particles that have been deposited into a polyaniline film. This polyaniline film was electropolymerizedon an e- beam deposited Au substrate from a 0.1 M solution of aniline in 1.0 M H2S04 to a coverage corresponding to -1.1 X 10-6molof repeat units/cm2. The coverage was determined by using linear sweep voltammetry (20 mV/s) from -0.2 to +0.9 V vs SCE and assuming the removal of one electron per repeat unit over this potential range.21 A fibrillar morphology is seen, which is typical of thick polyaniline films. Pt-Ru catalyst particles were incorporated into the polyaniline matrix by cycling (50 mV/s) the polyanilinemodified electrode between +0.5 and -0.3 V vs SCE in 0.5 M H2S04 containing -1.5 mM K2RuC150xH20and 3.3 mM K2PtCb2- for -55 min. For this sample we estimate deposition of 5.2 X lo4 mol/cm2( 1mg/cm2)of Pt, based on the integral of the cathodic charge passed in the deposition and assuming the charge is associated with the reduction of Pt(1V) to Pt(0). However, from XPS analysis it is clear that some Ru(II1) is reduced to Ru(O), so the actual amount of Pt deposited is less than the amount calculated. The Pt-Ru particles are similar in size (300-500nm in diameter) to what has been reported for Pt-Sn particles in a polyaniline matrix.l It is interesting to note that over the range of catalyst loadings studied (0.25-1.0 mg/cm2), the catalyst particle size appears to be nearly independent of the catalyst loading, indicating that during deposition the particles grow to some critical dimension where further particle growth stops. Further deposition of catalyst into the matrix leads to an increased number of particles but has little effect on particle size, so the catalyst particle size distribution remains small. The particles surround

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(21)Orata, D.; Butry, D. A. J. Am. Chem. SOC.1987,109,3574.

Hable and Wrighton

3286 Langmuir, Vol. 9,No. 11,1993

I

I

12 pm

Figure 1. SEM micrographs of a polyaniline/Pt-Ru assembly on a Au flag electrode. The polyaniline film was grown from a 0.1 M solution of aniline in 1.0 M H2S04 by cycling from -0.2 to +0.9 V vs SCE. The coverage is about 1.1X 10-6 mol/cm2. The catalyst was deposited from 0.5 M H2S04 containing 3.3 mM KzPtCk and 1.5 mM KzRuCkaH20 by cycling for -55 min between +0.5 and -0.3 V vs SCE. The catalyst loading is about 1mg/cm2.

the polymer fibrils in a fairly uniform manner and a minimal amount of clumping of the particles occurs near the surface of the sample. The density of particles is slightly higher a t the surface of the sample, but catalyst particles can be seen several micrometers into the sample. Figure 2 shows the results of XPS analysis of the polyaniline/Pt-Ru assembly shown in Figure 1. The key features of the XPS survey analysis are the C and N signals from the polyaniline and strong Pt and Ru signals from the catalyst particles. An 0 signal is present due to oxide

formation on the catalyst surface but may also indicate some decompostion of the polyaniline film.22 The XPS Ru 3d signal partially overlaps the C 1s signal of the polymer. However, with appropriate peak fitting the contribution of the two different elements can be determined and, in particular, the Ru 3d5/2 signal can be isolated and analyzed. The XPS analysis shows that the Ru is present in two different oxidation states; however, (22) Kobayashi, T.; Yoneyama, H.; Tamura,H. J. Electroanal. Chem. 1984,177,293.

Langmuir, Vol. 9, No. 11,1993 3287

Electrocatalytic Oxidation of MeOH and EtOH l0000~

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Potential, V vs. SCE Figure 3. Cyclic voltammetry of a polyaniline/Pt-Ru aseembly in 0.5 M HzSO4 the absence and presence of MeOH. The polymer was grown to a coverage of about 1.2 X 10-6 mol/cm2from 0.1 M aniline in 1.0 M HzSO4. The catalyst was deposited from 0.5 M HzSO4 containing 3.1 mM KzPtCle and 1.4 mM KaRuCb.zHz0. The catalyst loading is 0.5 mg/cm2. 20

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the exact assignment of the oxidation states is difficult. The lower binding energy 3d5p peak at 280 eV likely indicates the presence of Ru(0) on the The higher binding energy 3d5p peak at 281.4 eV might correspond to RuOrxHzO on the surface although there is a clear discrepancy in the literature on the binding energy of Ru 3d5p in The surface of RuOz may contain higher oxides.24 The binding energy also appears to be affected by the degree of hydration,24and in the case of the codeposited Pt-Ru the binding energy of the Ru peaks may be affected by interactions with adjacent Pt atoms. It should also be noted that the oxidation states present on the surface may be a function of the potential at which the sample was removed from solution. Analysis of the Ru 3p3p peak a t -462 eV yields no additional useful information. The surface of the catalyst particles in the sample in Figure 2 contains about 279% Ru or a PkRu ratio of about 3:l as determined by XPS. Previous workers have reported optimum enhancement of catalytic activity a t bulk composition of 15-33% R u . ~ * JIt~ is important to note that the composition may be considerably different at the surface compared to the bulk and the optimum ratio of PkRu may vary with different preparation techniques. Auger electron spectroscopy analysis of polyaniline/PtRu assemblies provides limited information about the

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(23) Lemay, G.; Kaliaguine, S.; Adnot, A.; Nahar,S.; Cozak, D.Can. J. Chem. 1986,64,1943. (24) Kim, K. S.; Winograd, N. J. Catal. 1974,35,66. (25) Folkesson, B. Acta Chem. Scand. 1973,27, 19. (26) Entina, V. S.; Petrii, 0. A. Sou. Electrochem. 1968,4, 97.

t

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0.5 1 .o 1.5 2.0 Concentration of K2RuCI5.xH20, g/L

Figure 4. Dependence of activity on the Ru content of the catalysts precursor solution for polyaniline/Pt-Ru catalysts.The polymer was grown from 0.3 M aniline. The KzPtCb concentration was kept constant at 3 mM, the polyaniline coverage was -1.2 X 1 P mol/cm2, and the catalyst loading was 0.5 mg/cm2. The catalytic activities are from cyclic voltammetry taken at 5 mV/s in 0.5 M H~SOIwith 20% by volume of added MeOH.

distribution of the Pt and Ru because the Pt and Ru signals show a substantial amount of overlap. Electrochemistry of Polyaniline/Pt-Ru Assemblies. Figure 3 shows current-voltage curves in the absence and presence of 20% MeOH in 0.5 M HzSO4 for a polyaniline-modified, glassy carbon electrode containing the Pt-Ru catalyst. The current-voltage profiie is similar to that for the Pt-Sn catalyst system. The onset of current for MeOH oxidation in the polyaniline/Pt-Ru assembly occurs at about 0 V vs SCE, and a maximum in MeOH oxidation current near +0.4 to +0.5 V vs SCE is typical of the Pt-Ru catalyst systems a t this coverage of polyaniline (1.2 X 1O-e mol/cm2)and catalyst loading (0.5 mg/ cm2). Above approximately +0.6 V vs SCE, the activity for MeOH oxidation is inhibited compared to Pt alone. Figure 4 shows the effect of varying the concentration of Ru(II1) in the precursor solution on catalytic activity of the Pt-Ru codeposit. With 3mM PtCl&good catalytic

,

3288 Langmuir, Vol. 9, No. 11,1993

16-

Hable and Wrighton

GC/Polyoniline/ P t - S n

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Figure 6. Effect of catalyst loading on activity for MeOH oxidation for a series of polyaniline/Pt-Sn electrodes. The polyaniline coverage was held constant at 1.2 X 108 mol/cm2. N

activity is observed over a concentration range of K2RuCl~rH20of about 0.5-2 mM. It is important to note that the ratio of PkRu for optimum catalytic activity will be different at different potentials. Comparison of Polyaniline/Pt-Sn and Polyaniline/ Pt-Ru Assemblies. Figures 5 and 6 show typical data for the dependence of current density on catalyst loading for MeOH oxidation at polyaniline-supported P t S n and Pt-Ru catalysts. These data show that the current increases nearly linearlywith increased amount of catalyst deposition, supporting the claim that increased catalyst deposition results in an increase in the number of catalyst particles and not an increase in size of the particles. The highest catalyst loading shown in Figure 6 (1mg/cm2) is the same as the catalyst loading for the polyaniline/PtRu assembly shown in Figure 1. It is also apparent from these data that the polyaniline/Pt-Ru system has an activity for MeOH that is similar to that of the polyaniline/ Pt-Sn system. Figures 7 and 8 show the effect of increased temperature on the activity for MeOH oxidation at polyaniline/Pt-Sn and polyaniline/Pt-Ru electrodes, respectively. Both Catalysts are significantlythermally activated, the activity increasing about 1order of magnitude at +0.3 V vs SCE from 0 to 50 OC. The catalyticactivitycontinues toincrease as the temperature is taken to the boiling point of MeOH, but the results are quite variable at higher temperatures probably due to degradation of the catalysts. A definite advantage of the Pt-Ru system is that the catalytic activity obtained from codeposition of Pt-Ru is

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more reproducible than for codeposition of Pt-Sn. The Pt-Sn catalyst activity can be quite variable even for samples prepared in an identicalmanner. This is reflected in the variability in the P t S n ratio from sample to sample for samplesprepared by the same method? The variability of the activity of the Pt-Sn assemblies is at least partly due to the reactivity of the SnC4.5H20 precursor used.

1

Langmuir, Vol. 9, No. 11, 1993 3289

Electrocatalytic Oxidation of MeOH and EtOH 64

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The SnC4.5H20 is known to hydrolyze rapidly and forms a colloid with the colloid particle size increasing over time until a precipitate forms.27 The stability of the Pt-Sn precursor solution can be increased by increasing the concentration of the Pt and Sn precursors, but even under these conditions the solutions remain clear for only several hours. In contrast the Ru precursor, K2RuClyxH20, is very stable in aqueousacid, and little or no apparent change in appearance occurs in these solutions even after several hours at ambient conditions. Although there is variability in activity for the Pt-Sn catalyst, there are only subtle differences in the current-voltage curves from the best Pt-Sn and Pt-Ru catalysts, the Pt-Sn catalyst having slightly higher activity at potentials negative of +0.3 V vs SCE and the Pt-Ru activity being somewhat better at more positive potentials. The codeposited Pt-Sn catalyst is different than the Pt-Ru catalyst with the Sn remaining in an oxidized state as an oxide, while in the case of the Pt-Ru catalyst some actual reduction of Ru(II1) to Ru(0) occurs. The nature of the Pt-Sn catalyst has been previously described? The evidence for reduction of Ru(II1) to Ru(0) comes from XPS analysis, uide supru, showing the presence of Ru(0) in the sample. A substantial difference between the polyaniline supported Pt-Sn and Pt-Ru catalysts can be seen in their activity for EtOH oxidation. Figure 9 compares the activity of a polyaniline/Pt, a polyaniline/Pt-Ru and a polyaniline/Pt-Sn assembly for MeOH oxidation and EtOH oxidation. The significant result shown in Figure 9 is that the enhancement in EtOH oxidation is even more dramatic than in the case of MeOH oxidation. Ethanol oxidation has been studied on Pt electrodes with Sn adatoms;2however, the use of electrodeposited catalysts in a conducting polymer offers the possibility of obtaining higher surface areas. In contrast to the polyaniline/PtSn electrodes the polyaniline/Pt-Ru electrodes show less of an improvement for EtOH oxidation compared to polyaniline/Pt electrodes, Figure 9. ~~~

(27) Feldstein, N.;Weiner, J. A.; Schnable,G. L. J.Electrochem.SOC. 1972,119, 1486.

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Figure 9. A comparisonof MeOH oxidation and EtOH oxidation

on polyaniline/Pt, polyaniliie/Pt-Ru, and polyaniline/Pt-Sn electrodes. The polyaniline coverage is 1 X 10-8 mol/cm2and the catalyst loading is 0.5 mg/cm2 for each electrode.

Figure 10 shows the temperature dependence for oxidation of EtOH on a polyaniline/Pt-Sn electrode. As in the case of MeOH oxidation the catalytic oxidation of EtOH is thermally activated. For oxidation of EtOH on polyaniline/Pt-Sn the onset for activity is negative of 0 V vs SCE, and a t this potential the polyaniline is essentially insulating12 and limits the current for EtOH Oxidation, particularly in the range of -0.2 to 0.0V vs SCE. Product analysis using 13CNMR indicates that EtOH oxidation at polyaniline/Pt-Sn electrodes yields initially acetaldehyde, as shown in eq 1. Unfortunately, acetalde-

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hyde oxidation a t polyaniline/Pt-Sn electrodes is not effective. This is remarkable since Pt-Sn modified carbon electrodes show significant activity for acetaldehyde (50 % ' by volume in 0.5 M H2S04) oxidation, to yield acetic acid, a t 0 V vs SCE where the same electrode shows significant activity for EtOH oxidation. The oxidation activity of Pt-Sn on carbon toward acetaldehyde is important, because the aldehyde provides two more electrons per starting molecule of EtOH and shows that polyaniline degradation, not Pt-Sn activity, limits the utility of polyaniline/Pt-Sn as a catalyst system for oxidation of the aldehyde. We have further demonstratedthe effect of acetaldehyde by using microelectrode arrays operating as electrochem-

Hable and Wrighton

3290 Langmuir, Vol. 9,No. 11,1993

eq 2, which is consistent with the fact that the Pt-Sn

-

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catalyst is active toward the oxidation of the intermediates, formaldehyde and formic acid.6 Indeed, these intermediates are oxidized at Pt-Sn at more negative potentials than MeOH itself. Although aqueous formaldehyde in 0.5 M HzS04, like acetaldehyde, degrades polyaniline, it is not evident that formaldehyde builds up to a high concentration since its formation occurs at a potential where it can be rapidly oxidized.

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0.6

Figure 10. Temperature dependence of EtOH oxidation for a polyaniline/Pt-Sn modified electrode.

ical transistors.12*2sPolyaniline was electrochemically deposited on microelectrode arrays to form a conductive pathway between two adjacent microwire electrodes. The current passing between the adjacent microelectrodes and through the polyaniline film was monitored before and after the addition of acetaldehyde to the 0.5 M HzS04 supporting electrolyte. In 0.5 M HzS04 the current was stable but decreased rapidly after the addition of 20% acetaldehyde. Returning the polyaniline-modified microelectrode array to pure 0.5 M HzSO4 did not result in an increase in the current flow between the electrodes, indicating irreversible damage to the polyaniline had occurred. Similar deleterious effects on polyaniline conductivity were also found for formaldehyde. Oxidation of MeOH at the polyaniline/Pt-Sn catalyst at +0.3 V does result (ultimately) in production of COz, (28) (a) Ofer, D.; Wrighton, M. S. J. Am. Chem. SOC.1988,110,4467. (b) Ofer, D.; Crooks, R. M.;Wrighton, M. S. J. Am. Chem. SOC.1990,112, 7869.

We have studied electrodes modified with polyaniline and Pt-Ru or Pt-Sn particles. The activity of the polyaniline/Pt-Ru or polyaniline/Pt-Sn assemblies for MeOH or EtOH oxidation is higher than that of polyaniline-coated electrodes modified with Pt alone. The polyaniline/Pt-Ru assemblies show structural features similar to those found previously for polyaniline/Pt-Sn assemblies.' Surface analysis shows Ru on the catalyst surface to be present in two different oxidation states, most likely Ru(0) and Ru(1V). The Pt-Ru catalyst was compared to the Pt-Sn catalyst which can alsobe deposited in a polyaniline matrix. For our methods of electrode modification the Pt-Ru and Pt-Sn catalysts show similar activity for MeOH oxidation. Both catalysts show a significant (and similar) temperature dependence for MeOH or EtOH oxidation. For EtOH oxidation the PtSn catalyst is superior to the Pt-Ru catalyst and the activity of Pt-Sn at low potentials, where polyaniline is nonconducting, is limited by the resistivity of the polyaniline film. Polyaniline also suffersas a catalyst support, because its conductivity is irreversibly degraded by the acidic solutions of acetaldehyde or formaldehyde.

Acknowledgment. We thank the Office of Naval Research and the Defense Advanced Research Projects Agency for support of this research.